1. Field of the Invention
This invention relates to heat exchangers. More particularly this invention relates to a heat exchanger core using expanded metal heat transfer surfaces for use in heat exchangers for cryogenic refrigeration systems.
This invention is related to the inventions disclosed and claimed in U.S. Pat. Nos. 3,477,504, issued Nov. 11, 1969 to Colyer and Fleming and 3,534,813 issued Oct. 20, 1970 to Fleming. These related patents are assigned as herein.
2. Prior Art
As indicated in the above-referenced U.S. Pat. No. 3,477,504, prior to the invention disclosed therein, heat exchangers of the tube type were commonly employed and constituted the prior art background of that invention. The above-referenced patents describe heat exchangers providing improved performance over the prior tube-type exchangers and comprising a plurality of perforated metal plates separated by thermally nonconductive separators in the core thereof. Subsequent to the development of these perforated plate heat exchangers, a second improved heat exchanger core construction was developed in which the perforated plate members are replaced by woven wire mesh members.
It is generally accepted that heat exchangers for use in cryogenic refrigeration systems should have very high thermal effectiveness and minimum size and weight. In order to provide very high thermal effectiveness, it is desired to have good flow distribution within the fluid streams, a low value of longitudinal heat conduction along the exchanger core, minimal (ideally no) fluid stream-to-fluid stream leakage, high heat transfer surface coefficients, and high heat transfer surface areas. In order to minimize weight, it is desirable to have heat transfer surfaces having substantial percentages of open area. It has been found that to maximize thermal effectiveness and minimize weight, approximately 70 percent open area is desirable in the heat transfer surfaces of a heat exchanger core. It is also desirable to provide a heat exchanger core construction which is structurally strong.
The perforated plate heat exchanger cores of the prior art are constructed by providing metal plates with perforations in a pattern corresponding to the fluid channels in the core and bonding the plates together with interleaved plastic separators of low conductivity, configured and aligned to provide the fluid flow channels. The perforated plate exchanger cores are therefore characterized by the abutting of a flat continuous metal surface with a flat continuous plastic surface. This construction provides for good fluid stream-to-fluid stream isolation and minimizes fluid leakage. On the other hand, the fabrication of perforated plate heat transfer surface elements results in higher costs than the use of wire mesh elements and has been found to be unable to economically practicably produce plates having more than 45 percent open area and therefore results in a heavier core. Moreover, the flat surface-to-flat surface metal-to-plastic bond characteristic of the perforated plate exchanger core has been found to produce insufficient structural bonding to maintain the structural integrity of the core after repeated cyclings without the use of additional compression elements to hold the plates and separators together. It was initially believed that such additional compression elements would not be necessary in perforated plate heat exchanger cores and in fact they were not found to be necessary on initial operation of perforated plate heat exchangers, however, after extensive hot-cold cycling, the differential thermal expansion between the heat transfer surface plates and the plastic separators was found to loosen the flat surface-to-flat surface bondings therebetween.
Wire mesh exchangers are less expensive to fabricate than perforated plate exchangers, typically provide the desired 70 percent open face area and consequently lower weight, and appear to provide improved structural bonding because of the ability of the separator material to at least partially penetrate the interstices of the portion of the mesh material between the separators. On the other hand, wire mesh exchangers have been found to suffer severe fluid stream-to-fluid stream leakage in some applications. Because the wire mesh is discontinuous even at crossing points and because the fine dimensions and complex geometry of the wire mesh prevent complete filling of interstices therein by plastic material, fine stream-to-stream leakage paths have proven unavoidable in practice. Since in some applications, stream-to-stream pressure differentials approximate 10 atmospheres, the isolation between channels of wire mesh heat exchanger cores is inadequate in those applications. This resulted in a degradation of thermal effectiveness of wire mesh heat exchangers in some applications to the extent that they are rendered completely unusable, which, of course, completely obviates their other advantages.